A conventional type of filter element is described in Japanese patent laid-open publication 60-87817. Namely, a conventional filter element is made of filter member which is bent to be zigzag shape and forms a generally ring-like shape. A filter member is so bent that the filter member makes a plurality of wavy portions, and the wavy portions are so arranged that a plurality of radial gaps are formed within adjacent wavy portions.
The radial gap is formed radially and some of which are connected with an inner space at the inner end thereof. The first radial gap being connected with the inner space is closed its outer end by the filter member. Some radial gaps are not connected with the inner space. Namely, the inner end of the second radial gap is closed by the filter member, and the outer end of the second radial gap opens to an outer atmosphere. The fluid is introduced into the second radial gaps and passes through the wavy portions toward the first radial gap which is connected with the inner space, so that the fluid introduced into the second radial gap flows toward the inner space through the first radial gap.
The conventional filter element, however, causes a large pressure drop when the fluid is introduced into the second radial gap and when the fluid flows from the first radial gap into the inner space. This pressure drop of the fluid flow reduces the efficiency of filtering and also reduce the life of the filter element.